ConTeXt: What is the equivalent of process_input_buffer callback?

Question

How can be LuaTeX callbacks used in ConTeXt? (with LuaTeX backend)
My main concern is the usage of process_input_buffer callback available in LuaTeX and LuaLaTeX. I am using it to altering the compiled source to reflect typografic rules customary in my country (maybe customary in general), to avoid one-letter prefixes ending the line.
Next MWEs should illustrate that:
One-letter prefix on the end of line (should be avoided):
documentclass{article}

usepackage{xcolor}

begin{document}
Filling text filling text filling text filling text filling text filling text textcolor{red}{filling V text}
end{document}

And correct version:
documentclass{article}

usepackage{xcolor}
usepackage{luacode}

begin{luacode}
function pevnaMezera(s)
  s = string.gsub(s, " V ", " V~")
  return s
end
end{luacode}

AtBeginDocument{%
  directlua{luatexbase.add_to_callback(
  "process_input_buffer", pevnaMezera , "pevnaMezera" )}
}

begin{document}
Filling text filling text filling text filling text filling text filling text textcolor{red}{filling V text}
end{document}

I am aware of this question: How to register a callback in ConTeXt; Howerver, presented answer seems focused only on callback pre_linebreak_filter, which is in ConTeXt defined as finalizer.
I wasnt able to find anything about finalizers in general in any ConTeXt documentation or on ConTeXt Garden.
NOTE: In actual projects I am using lua regexes in function for process_input_buffer callback, for brevity and readability I made simplier definition in MWE illustrating the problem.
EDIT3:
MWE changed according to comment of @Noone:
startluacode
userdata = userdata or {}

function userdata.pevnaMezera(s)
  s = string.gsub(s, " V ", " V~")
  return s
end

process_input_buffer = userdata.pevnaMezera
stopluacode

starttext
Filling text filling text filling text filling text filling text filling text filling text fil V text
stoptext

Is not adding non-breakable space. Is it correct from syntactical point of view?? I tryed to move the "assignment" to luacode block, but it did not work either.

Henri Menke · Accepted Answer

The actual question of how to suppress line breaks after single letter words is answered much better in this thread:
Hard spaces after one-letter words in ConTeXt

Before I begin to show how to register your own callbacks in ConTeXt, some words of warning (copied from my comment):

Do not use process_input_buffer to search and replace text! It is close to impossible to write down a pattern that is both sufficient and exhaustive at the same time. Also the error handling scenarios are frightening, because you either get an extremely obscure error that is very hard to debug or no error at all and silently garbled output.

With that out of the way, let's get to it.  ConTeXt by default sets up its own callback mechanism called processors and finalizers and “freezes” the original LuaTeX callbacks, i.e. makes them unusable and issues an error if you attempt to register one.  The reason for that is that ConTeXt registers own callbacks that are tightly coupled and breakage is expected if you tamper with them.  However, it is possible to unfreeze the callbacks by adding
enabledirectives[system.callbacks.permitoverloads]

to your document.  After that you can register callbacks as usual, e.g.
startluacode
callbacks.register("process_input_buffer", function(line)
    return line
end)
stopluacode

Note that in LMTX the callbacks are quite different from MkIV and some callbacks have been removed and other have been added.  The process_input_buffer callback for instance has been removed in LMTX.
After enabling the above directive, ConTeXt will greet you with this nice warning in the log:
system          >
system          > The callback system has been brought in an unprotected state. As a result of directly
system          > setting of callbacks subsystems of ConTeXt can stop working. There is no support for
system          > bugs resulting from this state. It's better to use the official extension mechanisms.
system          >

That should make it even more clear that this should not be used.

ConTeXt: What is the equivalent of process_input_buffer callback?

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